In an engine for a road vehicle, although the timing belt passed around a crank pulley and a cam pulley is entirely covered by a belt cover, the belts for auxiliary equipment such as an AC generator and a compressor for an air conditioner are normally exposed. However, in an outboard marine engine, it is desirable to cover the AC generator and the belt for transmitting power to the AC generator to protect them from moisture, and, for instance, Japanese patent laid open publication (kokai) No. 6-33790 discloses an arrangement for covering an AC generator.
The AC generator is typically placed on one side of the cylinder block of the engine at some distance from the engine. Therefore, the belt cover for covering the power transmission belt for the AC generator inevitably hangs over from the engine main body. To prevent such an overhanging belt cover from rattling or vibrating, it is necessary for the belt cover to have a sufficient rigidity. This contributed to an increase in the manufacturing cost.